1. Technical Field of the Invention
This invention relates generally to audio speaker driver motor structures, and more specifically to a motor structure having a voice coil with graduated windings in which some winding sections have a different electrical resistance and/or a different wire cross-sectional area per unit of coil height than others, which results in a varied BL along the voice coil winding height.
2. Background Art
FIG. 1 illustrates a conventional overhung speaker driver 10o such as is known in the prior art. The speaker driver includes a magnetically conductive pole plate 12 which includes a pole 16 which may be either coupled to or integral with the back plate 14 of the pole plate, as shown. The pole may include an axial hole 18 for permitting airflow to cool the motor structure and depressurize the inner portion of the diaphragm assembly. One or more ring-shaped permanent magnets 20 surround the pole, with a cavity 22 between the magnets and the pole. A magnetically conductive top plate 24o surrounds the pole, with a magnetic air gap 26 between the top plate and the pole. The pole plate, magnet(s), and top plate may collectively be termed a magnet assembly or a motor structure.
An electrically conductive, overhung voice coil 28o is rigidly attached to a cylindrical bobbin or voice coil former 30. The voice coil is suspended within the magnetic air gap such that, when energized with an electrical current, it will provide mechanical force to a diaphragm 32 which is coupled to the bobbin. When the energizing current which is passed through the voice coil is an alternating current, the voice coil moves up and down in the air gap along the axis of the speaker driver, causing the diaphragm to generate sound waves.
A frame 34 is coupled to the motor structure. Two suspension components couple the diaphragm assembly to the frame: a damper or spider 36 is coupled to the bobbin and the frame, and a surround 38 is coupled to the diaphragm and the frame. These two suspension components serve to keep the bobbin and diaphragm centered and aligned with respect to the pole, while allowing axial movement. A dust cap 40 seals the assembly and protects against infiltration of dust particles and other stray materials which might contaminate the magnetic air gap and thereby interfere with the operation or quality of the speaker driver.
When, as shown, the voice coil is taller (along the axis of the motor structure) than the magnetic air gap, the speaker driver is said to have an overhung geometry.
FIG. 2 illustrates a conventional underhung speaker driver 10u. The motor structure of the speaker driver includes permanent magnet(s) 20, a top plate 24u, and a pole 16u. The voice coil 28u is shorter (along the axis of the motor structure) than the magnetic air gap.
FIG. 3 illustrates a conventional equalhung speaker driver 10e. The motor structure of the speaker driver includes permanent magnet(s) 20, a top plate 24e, and a pole 16e. The voice coil 28e has substantially the same height (along the axis of the motor structure) as the magnetic air gap.
In any of these geometries, if the voice coil moves so far that there exists a different number of voice coil turns within the air gap (i.e. an overhung voice coil has moved so far that one end of it has entered the air gap, or an underhung voice coil has moved so far that one end of it has left the air gap, or an equalhung voice coil has moved), the speaker driver begins to exhibit nonlinear characteristics, and the sound quality is distorted or changed. This is especially problematic when producing low frequency sounds at high volume, which require long voice coil travel.
The common approach to solving this problem has been to use highly overhung or highly underhung geometries to achieve a high degree of linear voice coil travel. These approaches have inherent limitations, however. The highly overhung motor requires increasingly longer coils, which in turn increases the total moving mass of the diaphragm assembly and increases the DC resistance of the voice coil, assuming the same gauge wire is used. At some point, this ever-increasing mass becomes so great that the inherent mechanical stability design limits are reached, which prevents any further controllable increase in excursion. At the same time, increasing the voice coil mass with no resultant increase in utilized magnetic flux will reduce the overall efficiency of the speaker driver. Efficiency is proportional to BL squared, and inversely proportional to mass squared. Efficiency is also inversely proportional to the total electrical resistance of the voice coil. In the highly underhung geometry, other practical limits are reached because of the relative increase in magnet area required to maintain a constant B across a taller magnetic gap height in order to achieve higher linear excursions without sacrificing efficiency. Unfortunately, this increase in total magnetic flux (to achieve the same B) does not result in an increase in BL; in this case, the larger diameter magnets simply enable a taller magnetic air gap and greater Xmax, with no efficiency gain.
Presently, voice coils are wound with wire having the same cross-section (i.e. diameter) over the entire length of the voice coils, and with a substantially constant packing density over the voice coil height.
What is needed is an improved voice coil which provides increased travel and a less abrupt transition into non-linearity behavior.